Performance, catalysts

For the process developer knowledge of the following catalyst properties is important  

Aluminosilicate surfaces are classified as strong Bronsted acids, whereas silica gel is a weak acid. 

Give an explanation for the increased acidity when Al is present in the silicon dioxide lattice. 

Explain the cationic polymerization of alkenes on aluminosilicate surfaces with the aid of a reaction equation. 

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Ca.ta.lysts, Catalyst performance is the most important factor in the economics of an oxidation process. It is measured by activity (conversion of reactant), selectivity (conversion of reactant to desked product), rate of production (production of desked product per unit of reactor volume per unit of time), and catalyst life (effective time on-stream before significant loss of activity or selectivity). 

Catalyst performance depends on composition, the method of preparation, support, and calcination conditions. Other key properties include, in addition to chemical performance requkements, surface area, porosity, density, pore size distribution, hardness, strength, and resistance to mechanical attrition. 

Benefits depend upon location. There is reason to beheve that the ratio of hydrocarbon emissions to NO has an influence on the degree of benefit from methanol substitution in reducing the formation of photochemical smog (69). Additionally, continued testing on methanol vehicles, particularly on vehicles which have accumulated a considerable number of miles, may show that some of the assumptions made in the Carnegie Mellon assessment are not vahd. Air quaUty benefits of methanol also depend on good catalyst performance, especially in controlling formaldehyde, over the entire useful life of the vehicle. 

Resorcinol Derivatives. Aminophenols (qv) are important intermediates for the syntheses of dyes or active molecules for agrochemistry and pharmacy. Syntheses have been described involving resorcinol reacting with amines (91). For these reactions, a number of catalysts have been used / -toluene sulfonic acid (92), zinc chloride (93), zeoHtes and clays (94), and oxides supported on siUca (95). In particular, catalysts performing the condensation of ammonia with resorcinol have been described gadolinium oxide on siUca (96), nickel, or zinc phosphates (97), and iron phosphate (98). 

Dual-Pressure Process. Dual-pressure processes have a medium pressure (ca 0.3—0.6 MPa) front end for ammonia oxidation and a high pressure (1.1—1.5 MPa) tail end for absorption. Some older plants still use atmospheric pressure for ammonia conversion. Compared to high monopressure plants, the lower oxidation pressure improves ammonia yield and catalyst performance. Platinum losses are significantiy lower and production mns are extended by a longer catalyst life. Reduced pressure also results in weaker nitric acid condensate from the cooler condenser, which helps to improve absorber performance. Due to the spHt in operating conditions, the dual-pressure process requires a specialized stainless steel NO compressor. 

Although supported Pd catalysts have been the most extensively studied for butadiene hydrogenation, a number of other catalysts have also been the object of research studies. Some examples are Pd film catalysts, molybdenum sulfide, metal catalysts containing Fe, Co, Ni, Ru, Rh, Os, Ir, Pt, Cu, MgO, HCo(CN) on supports, and LaCoC Perovskite. There are many others (79—85). Studies on the weU-characteri2ed Mo(II) monomer and Mo(II) dimer on siUca carrier catalysts have shown wide variations not only in catalyst performance, but also of activation energies (86). 

The sulfonated resin is a close analogue of -toluenesulfonic acid in terms of stmcture and catalyst performance. In the presence of excess water, the SO H groups are dissociated, and specific acid catalysis takes place in the swelled resin just as it takes place in an aqueous solution. When the catalyst is used with weakly polar reactants or with concentrations of polar reactants that are too low to cause dissociation of the acid groups, general acid catalysis prevails and water is a strong reaction inhibitor (63). 

The hterature consists of patents, books, journals, and trade Hterature. The examples in patents may be especially valuable. The primary Hterature provides much catalyst performance data, but there is a lack of quantitative results characterizing the performance of industrial catalysts under industrially reaHstic conditions. Characterizations of industrial catalysts are often restricted to physical characterizations and perhaps activity measurements with pure component feeds, but it is extremely rare to find data characterizing long-term catalyst performance with impure, multicomponent industrial feedstocks. Catalyst regeneration procedures are scarcely reported. Those who have proprietary technology are normally reluctant to make it known. Readers should be critical in assessing published work that claims a relevance to technology. 

Surface Area. This property is of paramount importance to catalyst performance because in general catalyst activity increases as the surface area of the catalyst increases. However because some reaction rates are strongly dependent on the nature of the stmcture of the catalytic surface, a linear correlation of catalyst activity with surface area should not be expected. As the catalyst surface area increases, for many reactions the selectivity of the catalyst is found to decrease. If the support material is completely inert to the reactants and products, this effect may be diminished somewhat. 

The pore-size distribution and the nature of the pores in catalyst supports and hence the catalysts derived from them are important properties that significantly affect catalyst performance (16). In most cases, catalyst designers prefer an open-pore stmcture, that is, pores that have more than one opening, and a pore size as uniform as possible in order to obtain maximum utilization of the available pore volume. This can be achieved by careful choice of raw materials and processing conditions. 

The performance of a catalyst often depends as much on the care and method of preparation as on the identity of the active components. This fact has been learned by many who have failed to obtain reproducibiUty among catalyst preparations ia the laboratory or have been responsible for quaUty assurance ia catalyst manufacture. Also, there are many examples of strong effects of trace impurities ia raw material or catalyst support on catalyst performance. 

Composition. The results of elemental analyses are almost always included among the specifications for a commercial catalyst. Depending on the accuracy desired and whether or not the catalyst can be rendered soluble without great difficulty, elemental analysis may be performed by x-ray methods, by one of the procedures based on atomic absorption, or by traditional wet-chemical methods. Erequentiy it is important to determine and report trace element components that may have an effect on catalyst performance. 

Surface Area. Overall catalyst surface area can be determined by the BET method mentioned eadier, but mote specific techniques are requited to determine a catalyst s active surface area. X-ray diffraction techniques can give data from which the average particle si2e and hence the active surface area may be calculated. Or, it may be necessary to find an appropriate gas or Hquid that will adsorb only on the active surface and to measure the extent of adsorption under controUed conditions. In some cases, it maybe possible to measure the products of reaction between a reactive adsorbent and the active site. Radioactively tagged materials are frequentiy usehil in this appHcation. Once a correlation has been estabHshed between either total or active surface area and catalyst performance (particulady activity), it may be possible to use the less costiy method for quaHty assurance purposes. 

Performance Evaluation. Successful catalyst development requires a satisfactory means to determine the performance of the catalyst. The only significant proof of improvement in catalyst performance Hes in evaluation in a reactor under the proper conditions. 

The data most frequentiy collected and reported in catalyst performance evaluations are activity or turnover number, selectivity to the desired product(s), overall yield, catalyst life, and the identities and yields of by-products produced. These data are used to further catalyst or process development research efforts, to monitor catalyst manufacture, and to provide quaUty assurance information to catalyst users. 

Catalysts in this service can deactivate by several different mechanisms, but deactivation is ordinarily and primarily the result of deposition of carbonaceous materials onto the catalyst surface during hydrocarbon charge-stock processing at elevated temperature. This deposit of highly dehydrogenated polymers or polynuclear-condensed ring aromatics is called coke. The deposition of coke on the catalyst results in substantial deterioration in catalyst performance. The catalyst activity, or its abiUty to convert reactants, is adversely affected by this coke deposition, and the catalyst is referred to as spent. The coke deposits on spent reforming catalyst may exceed 20 wt %. 

In general, the following steps are used to ensure optimum regeneration and catalyst performance recovery alow temperature first pass to remove low boiling point hydrocarbons and other volatile matter, an initial combustion step to remove a portion of the sulfur and carbon, and a final combustion step to remove the remaining carbon to the target level. 

Esters such as ben2oates and phthalates are also used in the preparation of high activity catalysts for olefin polymeri2ation. They appear to function as electron donors in the catalyst complex, and play a significant role in catalyst performance (115). 

Since NO production depends on the flame temperature and quantity of excess air, achieving required limits may not be possible through burner design alone. Therefore, many new designs incorporate DENOX units that employ catalytic methods to reduce the NO limit. Platinum-containing monolithic catalysts are used (36). Each catalyst performs optimally for a specific temperature range, and most of them work properly around 400°C. 

Silver alone on a support does not give rise to a good catalyst (150). However, addition of minor amounts of promoter enhance the activity and the selectivity of the catalyst, and improve its long-term stabiHty. Excess addition lowers the catalyst performance (151,152). Promoter formulations have been studied extensively in the chemical industry. The most commonly used promoters are alkaline-earth metals, such as calcium or barium, and alkaH metals such as cesium, mbidium, or potassium (153). Using these metals in conjunction with various counter anions, selectivities as high as 82—87% were reported. Precise information on commercial catalyst promoter formulations is proprietary (154—156). 

Catalyst Function. Automobile exhaust catalysts are perfect examples of materials that accelerate a chemical reaction but are not consumed. Reactions are completed on the catalyst surface and the products leave. Thus the catalyst performs its function over and over again. The catalyst also permits reactions to occur at considerably lower temperatures. For instance, CO reacts with oxygen above 700°C at a substantial rate. An automobile exhaust catalyst enables the reaction to occur at a temperature of about 250°C and at a much faster rate and in a smaller reactor volume. This is also the case for the combustion of hydrocarbons. 

Beyond the catalytic ignition point there is a rapid increase in catalytic performance with small increases in temperature. A measure of catalyst performance has been the temperature at which 50% conversion of reactant is achieved. For carbon monoxide this is often referred to as CO. The catalyst light-off property is important for exhaust emission control because the catalyst light-off must occur rehably every time the engine is started, even after extreme in-use engine operating conditions. 

Sulfur oxides resulting from fuel sulfur combustion often inhibit catalyst performance in Regions II, III, and a portion of Region IV (see Fig. 7) depending on the precious metals employed in the catalyst and on the air/fuel ratio. Monolithic catalysts generally recover performance when lower sulfur gasoline is used so the inhibition is temporary. Pd is more susceptible than Rh or Pt. The last is the most resistant. Pd-containing catalysts located in hotter exhaust stream locations, ie, close to the exhaust manifold, function with Httie sulfur inhibition (72—74). 

ActivatedL yer Loss. Loss of the catalytic layer is the third method of deactivation. Attrition, erosion, or loss of adhesion and exfoHation of the active catalytic layer aU. result in loss of catalyst performance. The monolithic honeycomb catalyst is designed to be resistant to aU. of these mechanisms. There is some erosion of the inlet edge of the cells at the entrance to the monolithic honeycomb, but this loss is minor. The peUetted catalyst is more susceptible to attrition losses because the pellets in the catalytic bed mb against each other. Improvements in the design of the peUetted converter, the surface hardness of the peUets, and the depth of the active layer of the peUets also minimise loss of catalyst performance from attrition in that converter. 

One system for measuring catalyst failure is based on two oxygen sensors, one located in the normal control location, the other downstream of the catalyst (102,103). The second O2 sensor indicates relative catalyst performance by measuring the abiUty to respond to a change in air/fuel mixture. Other techniques using temperatures sensors have also been described (104—107). Whereas the dual O2 sensor method is likely to be used initially, a criticism of the two O2 sensors system has been reported (44) showing that properly functioning catalysts would be detected as a failure by the method. 

The emission control system for LPG is the same as is used for gasoline fueled engines with the exception of the fuel metering system. No evaporative emission system is required. Both Pt—Rh and Pd—Rh catalysts are good for emission control of LPG fuel exhaust. Pt provides the lowest light off temperature for C Hg. The sulfur content of LPG is also very low so that Pd catalysts perform very well. 

W. B. Williamson, H. S. Gandhi, and E. E. Weaver, ffects ofFuelA.dditiveMMT on Contaminent detention and Catalyst Performance, SAE 821193,... 

The AIG is used to uniformly inject diluted ammonia into the reactor. Uniform mixing of the ammonia into the flue gas is necessary to maintain catalyst performance at its highest level and to minimize ammonia leakage (ammonia sHp) past the catalyst. 

Design considerations and costs of the catalyst, hardware, and a fume control system are direcdy proportional to the oven exhaust volume. The size of the catalyst bed often ranges from 1.0 m at 0°C and 101 kPa per 1000 m /min of exhaust, to 2 m for 1000 m /min of exhaust. Catalyst performance at a number of can plant installations has been enhanced by proper maintenance. Annual analytical measurements show reduction of solvent hydrocarbons to be in excess of 90% for 3—6 years, the equivalent of 12,000 to 30,000 operating hours. When propane was the only available fuel, the catalyst cost was recovered by fuel savings (vs thermal incineration prior to the catalyst retrofit) in two to three months. In numerous cases the fuel savings paid for the catalyst in 6 to 12 months. 

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